Monitor diodes are used for output power control in semiconductor laser systems. They typically detect a portion of the light generated by the semiconductor laser and provide a corresponding signal to a control loop. The control loop then usually modulates the injection current level to the semiconductor laser chip to thereby modulate its output power.
Monitor diodes are relevant both to semiconductor pump lasers and semiconductor transmitter lasers. Transmitters are typically modulated either directly or have their cw output carrier modulated with a separate modulator such as a Mach-Zender interferometer. In contrast, pump lasers are used to optically excite a fiber gain medium in a fiber amplification system or other optically pumped gain medium.
In fact, pump lasers are one of the most common semiconductor laser devices in contemporary carrier-class systems. They are typically high power, edge-emitting device that pump either rare earth-doped gain fiber, such as erbium doped fiber, or regular fiber in a Raman pumping scheme. The output light levels can be critical, especially when trying to control gain tilt in rare earth-doped fiber systems or polarization balancing in Raman pumping. Further, the level of amplification and thus, pump light, are critical in optical add/drop devices to minimize or avoid the need for active optical attenuation devices.
Historically, the monitor diodes were positioned in the pump package to detect light exiting from the back facet. Back facets of semiconductor lasers are coated to have very high reflectivity. Nonetheless, some light still exits or leaks through the back facet and the level of this light can be used as a proxy for the level of light exiting though the front facet.
More recently, however, fiber Bragg grating stabilization has been required in main laser pump systems. This stabilization is required in many high capacity, dense wavelength division multiplexed DWDM systems to minimize pump mode-hopping noise in the amplified signals. In Raman pumping schemes, fiber Bragg grating stabilization is useful both to broaden the spectrum of light generated by the pump laser and to also promote polarization stability in the light from the pump lasers.
The dynamics of the system created by a fiber Bragg grating pump laser is somewhat more complex than the typical laser pump. Specifically, it has an external laser cavity defined by the distance from the semiconductor laser""s back facet to the fiber grating. This external cavity overlaps the cavity defined by the front and rear facets of the semiconductor device. The result is that the device has somewhat more complex operating characteristics. Specifically, in the context of monitoring diodes, many times the correspondence between the level of light exiting from the back and front facets may not always be stable with time and temperature. As a result, when the power control signal is derived solely from back facet light, the power stability of the pump lasers over time and ambient temperature is degraded
More recently, some pump lasers have used monitoring diodes that are oriented within the package to be more responsive to scattered or ambient light within the package. Specifically, they are installed flat on the submount so that their active areas are directed toward the package lid. As a result, the control signal generated by the monitoring diode is derived at least, in part, by scattered light in the package that originates from the front facet. This can improve their stability.
Experiments suggest that, even with the monitoring diode directed toward the package lid, the degree to which the monitoring diode""s signal tracks the output light is still suboptimal. This is due to the fact that the monitoring diode is still located behind the pump laser in a position to receive some back facet light and the orientation of the monitoring diode in combination with the optical absorption and charge migration characteristics of the typically silicon photodiodes still results in the photodiode being responsive, in large part, to back facet light.
The present invention is directed to a semiconductor laser system. Specifically, it includes a reflector on the lid that directs light emitted from the front facet to the monitoring diode. Thus, even when the diode is installed behind the semiconductor laser chip, and as a result receives back facet light, the ratio of front facet to back facet light received by the monitoring diode is increased due to the operation of the reflector.
In general, according to one aspect, the invention features a semiconductor laser system. It includes a sealed package including a lid, a floor, and sidewalls, which extend between the lid and floor. An edge-emitting semiconductor stripe laser chip is installed within the package. An optical fiber extends into the package via a feedthrough in one of the sidewalls. The optical fiber has an endface that is installed within the package, such that light is coupled between the chip and the endface through a front facet of the semiconductor chip. A monitoring diode is further installed within the package. Finally, according to the invention, a reflector is located on the lid, which reflector comprises a reflecting surface that is angled with respect to the plane of the lid to direct light emitted from the front facet to the monitoring diode either directly or indirectly.
The present invention is applicable regardless of the type of package used and thus can be applied to butterfly packages or dual in-line pin (DIP) packages, for example. Further, it can be applied to 980 or 1480 nanometer (nm) erbium fiber pumps or the 1300 to 1600 nm (14xc3x97xc3x97 nm) pumps used in Raman amplification. Normally, however, it is most applicable to Bragg grating stabilized lasers where tracking, based upon back facet light alone, is generally poor.
The present invention is most effective when fiber lenses are used to improve the coupling efficiency between the fiber endface and the semiconductor laser chip. Specifically, in the case of wedge-shaped and/or double angle wedge-shaped, cylindrical, quasi-cylindrical, or other aspheric fiber lenses, a substantial surface of the endface slopes obliquely toward the lid in a direction moving away from the laser chip. The angling results in light being directed toward the lid of the package, when it is not captured by the fiber lens/coupled into the fiber core. Specifically, according to the invention, a reflector on the lid, above the endface, is used to direct light to a photodiode installed behind the semiconductor chip.
The installation of the diode behind the chip along the device""s medial line is a preferred configuration for a number of engineering factors. Another place to install the monitoring diode is underneath the fiber in front of the chip or to a side of the chip. Physical conflicts with the installation of the fiber in the package typically need to be resolved, however. Further non-medial locations can be used. Typically, however, the medial configuration is desirable because of the emission characteristics of edge-emitting lasers, and how they distribute the non-coupled light in the package. Nonetheless, it should be noted that with the use of the reflector configuration, alternative configurations can be used without degrading performance.
According to one embodiment of the invention, the reflector comprises essentially a single reflecting surface that extends obliquely down from the lid and can be a specular or diffuse reflector.
In another embodiment, a Fresnel-type reflector is used that comprises a series of shorter reflecting surfaces. Such a Fresnel reflector can be attached to the lid, etched into it, or scored into the lid. In a further embodiment, a standard or blaze-type grating can be used.
In still a further embodiment, the photodiode can be installed on a pedestal, such that its active surface is located substantially above the top of the semiconductor chip. As a result, this orientation further diminishes the amount of back facet light detected by the monitoring diode.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of the invention may be employed in various and numerous embodiments without departing from scope of the invention.